Rigid polyurethane foam and process for producing the same

ABSTRACT

Rigid polyurethane foams produced by reacting a polyol component having a special composition with an isocyanate component in the presence of a blowing agent, a reaction catalyst and a foam stabilizer with a mold release time of as short as 5.5 minutes or less, have remarkably low thermal conductivity of 12.5×10 -3  Kcal/m.hr.°C. or less and a low density and are suitable as heat insulating materials for refrigerators and the like.

BACKGROUND OF THE INVENTION

This invention relates to a process for producing a rigid polyurethanefoam having a remarkably low thermal conductivity and a low density witha short mold release time, and the rigid polyurethane foam thusproduced.

Rigid polyurethane foams are usually obtained by reacting an polyolcomponent with an isocyanate component in the presence of a blowingagent, a reaction catalyst and a foam stabilizer. Since rigidpolyurethane foams are generally excellent in heat insulatingproperties, they are widely used as various kinds of heat insulatingmaterials but have many problems in moldability, uniformity in foaming,and the like. Various proposals have been made in order to improve theseproblems.

For example, Japanese Patent Unexamined Publication No. 58-134108proposes to use aromatic amine polyols mixed with a diluting polyolhaving a higher proportion of primary hydroxyl group and a lowviscosity, e.g. a polyol having a molecular weight of 600 or less andobtained by adding ethylene oxide to a polyhydric alcohol, in order toproduce rigid polyurethane foams with good moldability. Further,Japanese Patent Unexamined Publication No. 58-134109 proposes to usearomatic polyols terminals of which are capped with ethylene oxide asone component of polyol components in order to improve heat insulatingproperties by enhancing the uniformity of foam sizes of low densityrigid polyurethane foams. But the rigid polyurethane foams obtained bythe two above-mentioned processes of Japanese Patent UnexaminedPublications have a relatively large thermal conductivity of 13.6 to13.9×10⁻³ Kcal/m. hr. °C., which values are insufficient as an heatinsulating material for refrigerators.

As heat insulating materials for refrigerators, it is desired to have athermal conductivity of 12.5×10⁻³ Kcal/m. hr. °C. or less, morepreferably 12.0×10⁻³ to 11.0×10⁻³ Kcal/m. hr. °C., a low density and amold release time of a short time, preferably 5.5 minutes or less, morepreferably 5 minutes or less.

In order to meet the above-mentioned requirements, European Patent No.91,828 discloses a process for producing a rigid polyurethane foamwherein there is used as a polyol component a polyol mixture comprising(1) a tetrafunctional tolylenediamine polyol, (2) a bifunctionalpropylene polyol, (3) an octafunctional sucrose polyol and (4) atrifunctional diethanolamine polyol. The rigid polyurethane foamobtained by this process has an average foam diameter of 0.3 to 0.5 mm,a foam density (a panel foam density) of as low as 28 to 30 kg/m³ and amold release time of as short as 4 minutes, but a thermal conductivityis as slightly high as 13.0 to 15.0×10⁻³ Kcal/m.hr. °C., which value isinsufficient for refrigerators.

On the other hand, Japanese Patent Unexamined Publication No. 62-81414discloses a process for producing a rigid polyurethane foam having anaverage foam diameter of as fine as 0.1 to 0.2 mm, and a thermalconductivity of 11.0 to 12.0×10⁻³ Kcal/m.hr.°C. by using a polyolmixture containing methyl glucoside polyol. According to this process,the thermal conductivity is remarkably improved, but undesirably theskeleton strength of foam is weakened, the foam density (a panel foamdensity) is as large as 33 to 36 kg/m³ (10 to 20% is increased comparedwith the value of European Patent No. 91,828) due to very fine foamdensity, and the mold release time is 6 minutes or longer.

Therefore, it has long been desired to produce rigid polyurethane foamshaving a very low thermal conductivity and a low density in a short moldrelease time, suitable for use as a heat insulating material forrefrigerators.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a rigid polyurethane foamhaving a remarkably low thermal conductivity and a low density producedin a short mold release time, and a process for producing the same.

This invention provides a process for producing a rigid polyurethanefoam comprising reacting a polyol component with an isocyanate componentin the presence of a blowing agent, a reaction catalyst and a foamstabilizer, characterized in that the polyol component is a mixed polyolcomposition comprising

(a) 48 to 52% by weight of a polyol obtained by adding propylene oxideand ethylene oxide to tolylenediamine,

(b) 10 to 40% by weight of a polyol obtained by adding ethylene oxide tobisphenol,

(c) 13 to 17% by weight of a polyol obtained by adding propylene oxideto trimethylolpropane,

(d) 9 to 13% by weight of a polyol obtained by adding propylene oxide tosucrose, and

(e) 10 to 14% by weight of a polyol obtained by adding propylene oxideand ethylene oxide to diethanolamine, said mixed polyol compositionhaving an average OH value of 440 to 470.

This invention further provides a rigid polyurethane foam produced bythe above-mentioned process.

This invention still further provides a composition useful for producingthe rigid polyurethane foam used in the process mentioned above.

This invention also provides a use of the rigid polyurethane foam as aheat insulating material for refrigerators.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is characterized by using a special mixed polyolcomposition as the polyol component. By using such a special mixedpolyol composition, the mold release time (from the beginning of pouringa reaction mixture into a foaming mold to removing a foamed article fromthe foaming mold) can be shortened to 5 minutes or less, preferably 4.5minutes or less, the density of rigid polyurethane foam obtained can beas low as 33 kg/m³ or less, which value is for usual foams, preferably28 to 31 kg/m³ in terms of the panel foam density, the fluidity can beimproved remarkably (the difference between the free foam density andthe panel foam density can be made about 10 kg/m³ or less, preferably 8kg/m³ or less), and the thermal conductivity can be lowered to 12.5×10⁻³Kcal/m.hr. °C. or less, preferably 12.0 to 11.0×10⁻³ Kcal/m.hr.°C. orless.

The mixed polyol composition used in this invention comprises

(a) 48 to 52% by weight of a polyol obtained by adding propylene oxideand ethylene oxide to tolylenediamine diamine,

(b) 10 to 40% by weight of a polyol obtained by adding ethylene oxide tobisphenol such as bisphenol A,

(c) 13 to 17% by weight of a polyol obtained by adding propylene oxideto trimethylolpropane,

(d) 9 to 13% by weight of a polyol obtained by adding propylene oxide tosucrose, and

(e) 10 to 14% by weight of a polyol obtained by adding propylene oxideand ethylene oxide to diethanolamine,

said mixed polyol composition having an average OH value of 440 to 470.

In the mixed polyol composition, it seems that the component (a) is mosteffective for lowering the thermal conductivity, the component (b) iseffective for making the foams fine, the component (c) is effective formaking the foams finer and for improving the dimensional stability whenthe foam is low in the density, the component (d) is effective toimprove the mold release properties, and the component (e) is effectivefor improving the fluidity. When the proportions of the above-mentionedcomponents (a) to (e) are outside the above-mentioned range, the objectsof this invention cannot be attained.

Further, when the average OH value of the mixed polyol composition isless than 440, the dimensional stability is lowered, while when theaverage OH value is larger than 470, the friability readily takes place.Therefore, the average OH value should be 440 to 470 in order to producethe rigid polyurethane foam stably, considering that too high or too lowaverage OH value reduces productivity.

Considering the foam density, the thermal conductivity and the moldrelease time, the following mixed polyol composition is the mostpreferable:

(a) 50% by weight of a polyol obtained by adding propylene oxide andethylene oxide to tolylenediamine,

(b) 12% by weight of a polyol obtained by adding ethylene oxide tobisphenol,

(c) 15% by weight of a polyol obtained by adding propylene oxide totrimethylolpropane,

(d) 11% by weight of a polyol by adding propylene oxide to sucrose, and

(e) 12% by weight of a polyol obtained by adding propylene oxide andethylene oxide to diethanolamine, and the average OH value of polyolcomposition being 450.

These polyols mentioned above can be prepared by conventional processes.

As the isocyanate component which reacts with the polyol component,there can be used aromatic, aliphatic, alicyclic isocyanates, and thelike. Among them, the use of aromatic isocyanates is preferable.Examples of aromatic isocyanates are tolylene diisocyanate (TDI),diphenylmethane diisocyanate (MDI), polymethylene polyphenyl isocyanate(PAPI), naphthalene diisocyanate, xylylene diisocyanate, dianisidinediisocyanate, etc. Examples of non-aromatic isocyanates arehexamethylene diisocyanate, isophorone diisocyanate,methylene-bis(cyclohexyl isocyanate), etc. More preferable examples area mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate,4,4'-diphenylmethane diisocyanate, polymethylene polyphenyl isocyanatehaving an average functional number of 2.2 to 4.0, sucrose seriestolylene diisocyanate prepolymer, and the like aromatic polyisocyanates.

It is possible to use so-called modified isocyanates obtained bymodifying isocyanates with various methods and compounds.

Further, when a mixture of polymethylene polyphenyl diisocyanate and asucrose tolylene diisocyanate prepolymer, the content of the latterbeing preferably 30 to 70% by weight, more preferably 45 to 50% byweight, is used as the isocyanate component, the fluidity can beimproved without lowering the mold release properties, and further theskeleton strength of rigid foam is improved and the dimensionalstability is remarkably improved in the case of lowering the density ofthe rigid foam.

It is preferable to use the isocyanate component having the NCO percentof preferably 30 to 36%, more preferably 32 to 34%, from the viewpointof the balance of dimensional stability and fluidity.

The reaction ratio of the isocyanate component to the polyol componentis preferably 1.00 to 1.20, more preferably 1.10, in terms of the ratioof NCO of isocyanate to OH of polyol, i.e. NCO/OH.

A rigid polyurethane foam can be produced by reacting the polyolcomponent with the isocyanate component as fundamental startingmaterials in the presence of a blowing agent, a reaction catalyst, and afoam stabilizer.

As the blowing agent, there can be used water, a combination of waterand carbon dioxide gas or a compound which generates carbon dioxide, afluorocarbon blowing agent, air and the like inert gas. Examples of thefluorocarbon blowing agent are trichloromonofluoromethane,dichlorodifluoromethane, trichlorotrifluoroethane,dichlorotetrachloroethane, etc. When the fluorocarbon blowing agent isused as the blowing agent, it is used in an amount of 20 parts by weightor more, more preferably 30 to 60 parts by weight, based on 100 parts byweight of the polyol component.

The reaction catalyst used in preparing the foams of this invention maybe any of the catalysts known to be useful for this purpose, includingtertiary amines, organometallic salts, and mixtures of an organometallicsalt with one or more tertiary amines, the latter being preferred.Examples of the tertiary amines are triethylamine, triethylene diamine,trimethylamine, tetramethylene diamine, tetramethylbutane diamine,tetramethylhexamethylene diamine, N-methylmorpholine, N-ethylmorpholine,dimethylpiperazine, trimethylaminoethylpiperazine,dimethylaminooxadecane, dimethylcyclohexylamine, mixtures ofbis(dimethylaminoethyl ether) and dipropylene glycol such as 7:3 weightratio mixture, methyldicyclohexylamine, N-cyclohexylmorpholine,dimethylcyclohexylamine, methyldiethanolamine, mixtures ofdimethylcyclohexylamine and 2(3-pentyl)-1-dimethylaminocyclohexane,bis(dimethylaminoethylpropyl ether), mixtures of triethylene diamine anddipropylene glycol such as the 1:2 and 1:4 weight ratio mixtures,bis(dimethylaminopropyl ether), and mixtures of these catalysts. Thepreferred tertiary amine reaction catalysts are triethylenediamine,mixtures of triethylenediamine with dipropylene glycol, mixtures ofbis(dimethylaminoethyl ether) and dipropylene glycol,dimethylcyclohexylamine alone or as a mixture thereof with2-(3-pentyl)-1-dimethylaminocyclohexane. The tertiary amine reactioncatalyst is used in an amount of preferably 0.1 to 1.5 parts by weight,more preferably 0.25 to 0.75 part by weight per 100 parts by weight ofthe total polyol component.

Examples of the organo-metallic salts are salts of tin, titaniumantimony, aluminum, cobalt, zinc, bismuth, lead, and cadmium, the tinsalts, i.e., stannic and stannous salts, being preferred. Illustrativelysuch salts include the octoates, dilaurates, diacetates, dioctoates,oleates, and neodecanates of these metals, the octoates being preferred.The organometallic salt reaction catalyst is used in an amount ofpreferably about 0 to 0.5 part by weight, more preferably about 0.05 to0.2 part by weight, per 100 parts by weight of the total polyolcomponent. Particular when a mixed catalyst of a 33% dipropylene glycolsolution of triethylene diamine, trimethylaminoethylpiperazine, anethylene glycol solution of bis-2-dimethylaminoethyl ether, and formicacid is used, the fluidity and the mold release properties are furtherimproved and most preferable results can be expected.

As the foam stabilizer, there can be used conventionally used organicsilicone compounds, fluorine-containing surface active agents, etc.

For example, there can be used polyalkylene glycol silicone blockcopolymer represented by the formula: ##STR1## wherein Z₁ is CH₃ or--(CH₂)_(a) --(OC₂ H₄)_(b) --(OC₃ H₆)_(c) --OCH₃ ; Z₂ is --(CH₂)_(a)--(OC₂ H₄)_(b) --(OC₃ H₆)_(c) --OCH₃ ; a is an integer of 3; b is aninteger of 15 to 25; c is an integer of 5 to 15; x is zero or an integerof 1 to 7; and y is zero or an integer of 1 to 17. Among the compoundsof the formula (I), those having --(CH₂)₃ --(OC₂ H₄)₂₀ --(OC₃ H₆)₉--OCH₃ at both Z₁ and Z₂, x being 2 to 4 and y being 12 to 14 areparticularly preferable.

The foam stabilizer is preferably used in an amount of 0.1 to 5 parts byweight per 100 parts by weight of the total polyol component.

The composition for producing rigid polyurethane foams may furthercontain one or more conventional fire retardants, fillers, reinforcingfibers, colorants, and the like additives.

Rigid polyurethane foams can be produced by a one-shot process, asemi-prepolymer process, a prepolymer process, a spray process, and thelike. Among them, the one-shot process is preferable.

The foaming can be carried out by using a conventional foaming machine,for example, Pu-30 type foaming machine manufactured by Promat AG.Foaming conditions slightly change depending on the kind of foamingmachines used but usually are as follows:

liquid temperature: 18°-24° C.

charging pressure: 80-120 kg/cm²

charging amount: 15-30 kg/min

mold temperature: 35°-45° C.

More preferable foaming conditions are as follows:

liquid temperature: 20° C.

charging pressure: 100 kg/cm²

charging amount: 25 kg/min

mold temperature: 40° C.

The thus produced rigid polyurethane foams have a remarkably low thermalconductivity of 12.5×10⁻³ Kcal/m.hr.°C. or less, particularly12.0-11.0×10⁻³ Kcal/m.hr.°C. while keeping the density of the foams atusual low level, and the mold release time can be shortened to 5.5minutes or less, particularly 5 minutes or less, so that these rigidpolyurethane foams are remarkably excellent as a heat insulatingmaterial for refrigerators. Further, the rigid polyurethane foams canalso be used as heat insulating materials for other electric machinesand devices, building structures, vehicles, etc., or as heat insulatingmolded articles.

This invention is illustrated in detail by way of the followingExamples, in which all "parts" and "percents" are by weight unlessotherwise specified.

EXAMPLES 1 to 5, COMPARATIVE EXAMPLES 1 to 3

Using 100 parts of polyol components having an average OH value of 440to 470 as shown in Table 1 (PO=propylene oxide, EO=ethylene oxide), 0.5part of water (but 1.5 parts in Comparative Example 1), 2 parts ofpolyalkylene glycol silicone block copolymer of the formula (I) (Z₁=CH₃, x=3, y=13) (L-5340, a trade name, mfd. by Nippon Unikar Co.) as afoam stabilizer, isocyanate components as listed in Table 1(NCO/OH=1.10) wherein polymethylene polyphenyl diisocyanate (NCO%=31)and sucrose tolylene diisocyanate prepolymer(NCO%=35) were used inamounts as listed in Table 1, 4 parts of a catalyst mixture comprising a33% dipropylene glycol solution of triethylene diamine,trimethylaminoethylpiperazine, and dimethylaminooxadecane, and 43 to 46parts of trichloromonofluoromethane (R-11, a trade name, mfd. by E. I.du Pont de Nemours & Co.) as a blowing agent, foaming and curing wereconducted to test physical properties as listed in Table 1.

In Table 1, the physical properties were obtained as follows:

Free foam density (a):

A density of a foam obtained by foaming in a container of 200×200×200 mmin inside dimensions made of a veneer board (kg/m³).

Panel foam density (b):

A density of foam obtained by foaming at 40° C. in a container of400×600×35 (thickness) mm in inside dimensions made of aluminum (kg/m³).

Fluidity: (b)-(a)

Dimensional stability:

Change (%) in thickness direction of a panel foam of 450×650×35(thickness) mm after standing at -20° C. for 24 hours.

Thermal conductivity:

Measured by using Anacon 88 type (mfd. by Anacon Co.) and a panel foamof 200×200×50 (thickness) mm at an average temperature of 23.8° C.

Mold release time:

A time from the beginning of pouring of a liquid composition to a moldof 400×1100×65 (thickness) mm in inner dimensions made of aluminum tothe removing of the resulting foam from the mold. Foaming was conductedat 40° C. As a measure for releasable foaming, the growth at thethickness direction of a foam was made 2 mm or less.

                                      TABLE 1                                     __________________________________________________________________________                                  Comparative Example                                                                      Example                                      Example No.           1   2  3   1  2  3  4  5                        __________________________________________________________________________    Polyol  Tolylenediamine + PO/EO (%)                                                                         65  50 50  50 50 50 50 50                               Bisphenol A + EO (%)  --  12 12  12 12 12 12 12                               Methyl glycomide + PO (%)                                                                           --  15 15  -- -- -- -- --                               Trimethylolpropane + PO (%)                                                                         --  -- --  15 15 15 15 15                               Diethanolamine + PO/EO (%)                                                                          6   12 12  12 12 12 12 12                               Sucrose + PO (%)      14  11 11  11 11 11 11 11                               Propylene glycol + PO (%)                                                                           15  -- --  -- -- -- -- --                       Inocyanate                                                                            Polymethlene polyphenyl diisocyanate (%)                                                            100 70 70  70 60 55 50 45                               Sucrose tolylene diisocyanate prepolymer (%)                                                        --  30 30  30 40 45 50 55                       Blowing agent                                                                         Trichloromonofluoromethane (parts)                                                                  46  43 46  46 46 46 46 46                       Physical                                                                              Free foam density (a) (kg/m.sup.3)                                                                  21.5                                                                              24.3                                                                             23.2                                                                              23.0                                                                             22.8                                                                             22.6                                                                             22.5                                                                             22.5                     properties                                                                            Panel foam density (b) (kg/m.sup.3)                                                                 29.2                                                                              34.3                                                                             32.8                                                                              32.6                                                                             32.2                                                                             32.0                                                                             31.9                                                                             31.8                             Fluidity (b)-(a)      7.7 10.0                                                                             9.6 9.6                                                                              9.4                                                                              9.4                                                                              9.4                                                                              9.3                              Dimensional stability (%)                                                                           -0.5                                                                              -0.6                                                                             -3.0                                                                              -2.3                                                                             -2.0                                                                             -1.5                                                                             -1.5                                                                             -1.2                             Thermal conductivity (10.sup.-3 Kcal/m · hr.                                               13.5ree.C.)                                                                       11.5                                                                             12.0                                                                              11.7                                                                             11.8                                                                             11.8                                                                             11.9                                                                             11.9                             Mold release time (min)                                                                             4.5 6  5.5 5  5  5  5  5                        __________________________________________________________________________

As is clear from Table 1, in Comparative Example 1 wherein a polyolobtained by adding ethylene oxide to bisphenol A and a polyol obtainedby adding propylene oxide to trimethylolpropane are not contained, themold release time is as good as 4.5 minutes but the thermal conductivityis as high as 13.5×10⁻³ Kcal/m.hr.°C., which value is inferior to thepresent invention. In Comparative Examples 2 and 3 wherein there is useda polyol obtained by adding propylene oxide to methylene glycoside inplace of a polyol obtained by adding propylene oxide totrimethylolpropane, the thermal conductivity is improved to11.5-12.0×10⁻³ Kcal/m.hr.°C., but the mold release time is undesirablyas long as 6 to 5.5 minutes.

In contrast, in Examples 1 to 5, the thermal conductivity is as low as12×10⁻³ Kcal/m.hr.°C. and the mold release time is as good as 5 minutes.Further, the panel foam density is about 32 kg/m³, which value satisfiesthe requirement of low density. Further, in the cases of Examples 3 and4, wherein the content of sucrose tolylene diisocyanate prepolymer is 45to 50% in the total isocyanate components, the panel foam density islowered preferably to 31.9 to 32.0 kg/m³.

As a result, as shown in Example 3, the following combination ofstarting materials is the most preferable from the viewpoint of balanceof fluidity, dimensional stability, thermal conductivity and moldrelease properties:

    ______________________________________                                                       tolylene diamine + PO + EO                                                                           50%                                                    bisphenol A + EO       12%                                     Polyol         trimethylolpropane + PO                                                                              15%                                                    sucrose + PO           11%                                                    diethanolamine + PO + EO                                                                             12%                                                    polymethylene polyphenyl diisocyanate                                                                55%                                     Inocyanate     sucrose tolylene diisocyanate                                                                        45%                                                    prepolymer                                                     ______________________________________                                    

EXAMPLES 6 to 13

There were used as the polyol, the same mixed polyol composition as usedin Example 3 in an amount of 100 parts, water in an amount of 0.5 part,polyalkylene glycol silicone block copolymer as shown in Table 2 in anamount of 2 parts as a foam stabilizer, as the catalyst, the samecatalyst (A) as used in Examples 1 to 5 in an amount of 4 parts, or amixed catalyst (B) comprising a 33% dipropylene glycol solution oftriethylene diamine, trimethylaminoethylpiperazine, an ethylene glycolsolution of bis-2-dimethylaminoethyl ether, and formic acid in an amountof 3.5 parts, as the blowing agent, trichloromonofluoromethane (R-11) inan amount of 44-46 parts, the same mixed isocyanate as used in Example 3(NCO/OH=1.10), and the foam stabilizer as shown in Table 2, and foamingand curing were carried out in the same manner as described in Examples1 to 5.

The results are as shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                   Example No.                                                                   6   7   8   9   10  11 12   13                 __________________________________________________________________________    Foam stabilizer *1                                                                     ##STR2##            x y                                                                             0 12                                                                              2 12                                                                              4 12                                                                              6 12                                                                              2 10                                                                              2 12                                                                              2 14                                                                              2 16               Catalyst *2                                                                           Mixed catalyst (A) or (B)                                                                            (A) (A) (A) (A) (B) (B) (B) (B)                Blowing agent                                                                         Trichloromonofluoromethane (parts)                                                                   46  46  46  46  44  44  44  44                 Physical                                                                              Free foam density (a)(kg/m.sup.3)                                                                    22.4                                                                              22.3                                                                              22.3                                                                              22.2                                                                              22.2                                                                              22.0                                                                              21.9                                                                              21.9               properties                                                                            Panel foam density (b)(kg/m.sup.3)                                                                   31.0                                                                              30.5                                                                              30.5                                                                              30.4                                                                              30.0                                                                              29.4                                                                              29.3                                                                              29.3                       Fluidity (b) - (a)     8.6 8.3 8.2 8.2 7.8 7.4 7.4 7.4                        Dimensional stability (%)                                                                            -0.5                                                                              -0.7                                                                              -0.7                                                                              -2.0                                                                              -0.5                                                                              -0.6                                                                              -0.6                                                                              -1.7                       Thermal conductivity (10.sup.-3 Kcal/m · hr ·               °C.)            12.3                                                                              11.6                                                                              11.5                                                                              11.5                                                                              12.1                                                                              11.6                                                                              11.6                                                                              11.5                       Mold release time (min)                                                                              4.5 4.5 4.5 4.5 4   4   4   4.5                __________________________________________________________________________    Note                                                                          *1: Z.sub.1 and Z.sub.2 are each (CH.sub.).sub.3(OC.sub.2 H.sub.4).sub.20(    OC.sub.3 H.sub.6).sub.9OCH.sub.3                                              *2                                                                              Mixed catalyst (A):                                                           33% Dipropylene glycol solution of triethylenediamine                                                        50%                                            Trimethylaminoethylpiperazine  15%                                            Dimethylaminooxadecane         35%                                            Mixed catalyst (B):                                                           33% Dipropylene glycol solution of triethylenediamine                                                        20%                                            Trimethylaminopiperazine       20%                                            Ethylene glycol solution of bis-2-dimethylaminoethyl ether                                                   50%                                            Formic acid                    10%                                      

As is clear from Table 2, when the polyalkylene glycol silicone blockcopolymer of the formula: ##STR3## wherein Z₁ and Z₂ are each --(CH₂)₃--(OC₂ H₄)₂₀ --(OC₃ H₆)₉ OCH₃ ; x is an integer of 2 to 4; and y is aninteger of 12 to 14 is used as in Examples 7, 8, 11 and 12, the thermalconductivity becomes 11.6×10⁻³ Kcal/m.hr.°C. or less, the mold releasetime becomes 4.5 minutes or less and the panel foam density is furtherlowered to 30.5 kg/m³ or less, so that more excellent effects areobtained compared with those of Examples 1 to 5.

Further, when the mixed catalyst (B) is used, it is revealed that thefluidity is further improved, (the difference between the panel foamdensity (b) and the free foam density (a) being 7.4 kg/m³), the panelfoam density becomes 30 kg/m³ or less, and the mold release time becomes4.5 minutes or less, these effects being more preferable.

As shown in the above Examples, it is possible to produce rigidpolyurethane foams having remarkably low thermal conductivity and lowdensity with a very short mold release time, said rigid polyurethanefoams satisfying the same foam properties as those of conventional rigidpolyurethane foams by properly combining the special mixed polyolcomposition, the special isocyanate proportions and the special foamstabilizer. Particularly, as shown in Examples 7, 8, 11 and 12, thedifference between the free foam density and the panel foam density islessened to show by far better fluidity than conventional foams, so thatmore effects can be exhibited at the time of pouring the startingmaterial liquid into a mold practically.

As mentioned above, according to this invention, the density of theresulting foams can be lowered by 10% or more compared with conventionallow thermal conductivity foams, while maintaining the thermalconductivity of the foams as remarkably low as 0.0125 Kcal/m.hr.°C. orless, more preferably 0.0110 to 0.0120 Kcal/m.hr.°C. In other words,there can be obtained rigid polyurethane foams having almost the samedensity of 30 kg/m³ (in terms of the panel foam density) as conventionalfoams (the thermal conductivity being as poor as 0.0135 to 0.0150Kcal/m.hr.°C.), so that it is possible to attain energy saving and spacesaving by lessening the thickness of heat insulating materials, whichresults in lowering the production cost while maintaining the excellenteffects as mentioned above. Further, since the density of foam ismaintained at a low level, the foaming pressure is lessened and the moldrelease time can be shortened remarkably.

What is claimed is:
 1. A process for producing a rigid polyurethane foamwhich comprises reacting a polyol component with an isocyanate componentin the presence of a blowing agent, a reaction catalyst and a foamstabilizer, said polyol component being a mixed polyol compositioncomprising(a) 48 to 52% by weight of a polyol obtained by addingpropylene oxide and ethylene oxide to tolylenediamine, (b) 10 to 40% byweight of a polyol obtained by adding ethylene oxide to bisphenol, (c)13 to 17% by weight of a polyol obtained by adding propylene oxide totrimethylolpropane, (d) 9 to 13% by weight of a polyol obtained byadding propylene oxide to sucrose, and (e) 10 to 14% by weight of apolyol obtained by adding propylene oxide and ethylene oxide todiethanolamine, said mixed polyol composition having an average OH valueof 440 to
 470. 2. A process according to claim 1, wherein the polyolcomponent and the isocyanate component are used in a reaction ratio ofNCO/OH=1.00 to 1.20.
 3. A process according to claim 1, wherein theisocyanate component is a mixture of 50 to 55% by weight ofpolymethylene polyphenyl diisocyanate and 45 to 50% by weight of sucrosetolylene diisocyanate prepolymer, said mixture having NCO percent of 32to
 34. 4. A process according to claim 1, wherein the foam stabilizer isa polyalkylene glycol silicone block copolymer represented by theformula: ##STR4## wherein Z₁ and Z₂ are independently --(CH₂)_(a) --(OC₂H₄)_(b) --(OC₃ H₆)_(c) --OCH₃ ; a is an integer of 3; b is an integer of15 to 25; c is an integer of 5 to 15; x is an integer of 2 to 4; and yis an integer of 12 to
 14. 5. A rigid polyurethane foam produced by theprocess of claim
 1. 6. A rigid polyurethane foam produced by the processof claim
 2. 7. A rigid polyurethane foam produced by the process ofclaim
 3. 8. A rigid polyurethane foam produced by the process of claim4.
 9. A mixed polyol composition for producing a rigid polyurethane foamcomprising(a) 48 to 52% by weight of a polyol obtained by addingpropylene oxide and ethylene oxide to tolylenediamine, (b) 10 to 40% byweight of a polyol obtained by adding ethylene oxide to bisphenol, (c)13 to 17% by weight of a polyol obtained by adding propylene oxide totrimethylolpropane, (d) 9 to 13% by weight of a polyol obtained byadding propylene oxide to sucrose, and (e) 10 to 14% by weight of apolyol obtained by adding propylene oxide and ethylene oxide todiethanolamine, said mixed polyol composition having an average OH valueof 440 to
 470. 10. A mixed polyol composition according to claim 9,wherein said composition comprises(a) 50% by weight of a polyol obtainedby adding propylene oxide and ethylene oxide to tolylenediamine, (b) 12%by weight of a polyol obtained by adding ethylene oxide to bisphenol,(c) 15% by weight of a polyol obtained by adding propylene oxide totrimethylolpropane, (d) 11% by weight of a polyol obtained by addingpropylene oxide to sucrose, and (e) 12% by weight of a polyol obtainedby adding propylene oxide and ethylene oxide to diethanolamine, and theaverage OH value of polyol composition being
 450. 11. A composition forproducing a rigid polyurethane foam comprising(A) an isocyanatecomponent, (B) a polyol component comprising (a) 48 to 52% by weight ofa polyol obtained by adding propylene oxide and ethylene oxide totolylenediamine, (b) 10 to 40% by weight of a polyol obtained by addingethylene oxide to bisphenol, (c) 13 to 17% by weight of a polyolobtained by adding propylene oxide to trimethylolpropane, (d) 9 to 13%by weight of a polyol obtained by adding propylene oxide to sucrose, and(e) 10 to 14% by weight of a polyol obtained by adding propylene oxideand ethylene oxide to diethanolamine, said mixed polyol compositionhaving an average OH value of 440 to 470, the reaction ratio of (A)/(B)being 1.00 to 1.20 in terms of NCO/OH value, (C) 20 parts by weight ormore of a blowing agent per 100 parts by weight of the total polyolcomponents, and (D) 0.1 to 5 parts by weight of a foam stabilizer per100 parts by weight of the total polyol components.
 12. A compositionaccording to claim 11, wherein the isocyanate component is a mixture of50 to 55% by weight of polymethylene polyphenyl diisocyanate and 45 to50% by weight of sucrose tolylene diisocyanate prepolymer, said mixturehaving NCO percent of 32 to
 34. 13. A composition according to claim 11,wherein the foam stabilizer is a polyalkylene glycol silicone blockcopolymer represented by the formula: ##STR5## wherein Z₁ and Z₂ areindependently --(CH₂)_(a) --(OC₂ H₄)_(b) --(OC₃ H₆)_(c) --OCH₃ ; a is aninteger of 3; b is an integer of 15 to 25; c is an integer of 5 to 15; xis an integer of 15 to 4; and y is an integer of 12 to
 14. 14. A processfor using the rigid polyurethane foam of claim 5 as an heat insulatingmaterial for refrigerators.